68 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
much less for violet and ultra-violet rays than for red and infra-red ones, we 
probably get infra-red rays from deeper-lying and hence hotter layers in the 
sun than we do ultra-violet ones. é 
We conclude that the solar radiation comes from sources ranging in tem- 
perature perhaps between the limits 5,000° and 7,000° absolute centigrade, but 
mostly from sources between 6,000° and 7,000°. 
Washington observations—Further experiments have been made, 
under Mr. Fowle’s direction, on the transmission of radiation of great 
wave lengths through long columns of air containing known qualities 
of water vapor. Many of these observations are not yet reduced, 
so that it is not yet proper to give a numerical summary of 
results. The length of the column experimented upon has been 
increased to 800 feet. The measurements cover the infra-red 
spectrum, from the A line to a wave length of about 17p. 
The observations of the water contents of the air column 
are made by means of pairs of wet and dry thermome- 
ters located at a number of points along the path. 
The air is thoreughly stirred before readings. Check 
experiments by Mr. Aldrich, in which he drew the 
air through phospherus pentoxide tubes and 
weighed the water absorbed, have confirmed 
the accuracy of the water-vapor determina- 
tions. Mr. Fowle has made a preliminary 
comparison of the upper infra-red spec- 
trum bands p, o, 7, ® WV, and ©, as 
observed through the tube with 
the same bands as observed 
through the whole atmosphere 
at Washington, Mount Wiison, 
and Mount Whitney. The re- 
sults are most interesting, though 
not yet ripe for publication, and 
will probably lead to more exact 
knowledge of the total quantity 
of water vapor in the atmos- 
phere, and its variation with 
the aititude of the observer and 
the season of the year. 
Reduction of observations.— 
Upward of 100 days of solar- 
constant measurements have 
been made on Mount 
Wilson on each of the 
last several years. Each 
day requires the equiva: 
Fic. 3.—Abbot silver disk pyrheliometer. Jent of three full days of 
computation. This work 
is being done at Washington by Messrs. Fowle and Aldrich and Miss Graves 
and certain graphical parts of it by minor clerk Segal. The solar-constant 
reductions are computed as far as the middle of the observing season of 1910. 
Pyrheliometry.—Additional comparisons of the Mount Wilson secondary pyr- 
heliometers have been made with primary standard pyrheliometer No. 3. These 
are not yet all reduced, but such as have been finished confirm the results of 
the previous fiscal year, so that we may regard the scale of absolute pyrheli- 
